This invention relates to a magnetic resonance imaging (MRI) apparatus and, more particularly, an MRI apparatus which employs a gradient coil system to provide a gradient magnetic field.
An MRI apparatus detects magnetic resonance signals from nuclei (e.g. proton) in an object and reconstructs a tomographic image of a desired slice position of the object. An MRI apparatus holds a great advantage in medical use because of non-invasiveness or non X-ray dosing.
In some imaging methods applied to an MRI apparatus, such as a rapid imaging or an angiographic imaging, a gradient field with a great intensity is rapidly switched in an interval of several milli-seconds, for example. Such imaging methods are disclosed in, for example, U.S. Pat. No. 4,165,479, issued Aug. 21, 1979 to Mansfield or U.S. Pat. No. 4,516,075, issued May 7, 1985 to Moran. Extremely high power is required from a gradient amplifier for supplying electrical power to a gradient coil in such imaging methods.
Usui's Japanese patent application No. 62-268134 discloses a gradient coil system which enables gradient amplifiers to provide a great intensity of gradient field with less load. Usui's gradient coil system comprises a plurality of gradient coil elements respectively connected to individual gradient amplifiers.
FIG. 1 shows a gradient coil system for an MRI apparatus, which is of a type such as disclosed in Usui. A gradient coil system 1 comprises a gradient coil unit 6 including a plurality of gradient coil elements 2a to 2e and gradient amplifiers 3a to 3e respectively connected thereto.
FIG. 2A shows a developed coil pattern of the gradient coil elements 2a to 2c in FIG. 1, omitting elements 2d and 2e in the drawing for simplification. The gradient coil elements 2a to 2c are formed on a substrate 4. In the coil system comprising a plurality of coil elements, called a "multi-filar" type, gradient amplifiers with great power are not required since total intensity of the gradient field produced by the coil system is shared by each of the coil elements.
However, the multi-filar gradient coil system shown in FIG. 2A has complicated coil patterns and it may take a long time to manufacture it especially when the coil patterns are hand-wired.
Further, in the coil system shown in FIG. 2A, lead portions 5 to respectively transmit power therethrough from the gradient amplifiers 3a to 3c to the gradient coil elements 2a to 2c cross the coil turns of the coil elements 2a to 2c. This may also make the manufacturing process of the system complicated when the coil patterns are formed either by hand-wiring or etching.
Especially in manufacturing the coil system shown in FIG. 2A by etching, a further problem may arise. FIG. 2B shows a cross section along the line B--B in FIG. 2A. It is necessary that the pitch between the adjacent coil turns be small such that a large number of coil elements can be formed on the substrate 4, while each turn of the coil is required to have a predetermined cross-sectional area (width B.times.height H) to conduct a predetermined intensity of current. For example, the height H is 2 mm when the width B is 12 mm and the pitch P is 2 mm. On the other hand, when the height of an etched conductor is too large, the problem of "under-etching", which causes some undesirable conductors to remain nearby an etched conductor, may occur. It may be difficult to keep the desired pitch P and may cause the coil pattern to short-circuit.
Therefore, a limited number of coil elements can be formed on the substrate 4.
Further, when the width B of a conductor line of a coil is small, the conductor must have a large cross-sectional area, even larger than an area sufficient to conduct a predetermined intensity of current, to keep a predetermined heat-radiation efficiency since the surface area substantially effective to radiate the heat from the conductor line is small. A cooling device for the gradient coil unit, for example, a water-cooling system, may be required.
Furthermore, when a shield coil is provided outside the gradient coil as disclosed in U.S. Pat. No. 4,733,189, issued Mar. 22, 1988 to Punchard et al. and the shield coil is formed in multi-filar type, the same problems as stated above may occur.
Furthermore, when a large number of shim coils for minimizing the inhomogeneity of the static magnetic field are provided inside a magnet to obtain a precise image, the same problems as stated above may occur.